1. Field of the Invention
The invention is directed to a composite optical amplifier, and in particular, to a composite optical amplifier that includes a Raman fiber amplifier and semiconductor optical amplifier.
2. Technical Background
The continuous growth of bandwidth requirements in optical-based communication systems has resulted in a large demand for systems able to operate outside the amplification band provided by Erbium-doped fiber amplifiers. Erbium-doped fiber amplifiers effectively operate over a limited wavelength band. Depending on amplifier configuration and fiber composition, Erbium-doped fiber can be used for amplification in a wavelength band extending from 1530 nm to 1620 nm, although at least two different erbium-doped fiber amplification configurations would be required to cover this entire range.
Other rare earth-doped fiber amplifiers have been used for amplification outside the erbium wavelength band from 1530 nm to 1620 nm. These other rare earth-doped amplifiers include Thulium-doped amplifiers operating from 1440 nm to 1510 nm, Praseodymium amplifiers operating from 1250 nm to 1310 nm and Neodymium amplifiers operating from 1310 nm to 1350 nm. Each of these rare earth-doped amplifiers exhibit very low efficiency as well as other technical problems associated with each particular kind of dopant when compared to Erbium-doped amplifiers.
Rare earth-doped amplification systems cover much of the available transmission window of traditional silica fiber. However, this transmission window has been expanded with the development of new fibers. In many new fibers, where the OH absorption around 1400 nm has been greatly reduced, there is a potential for optical amplifier configurations which can amplify between an entire optical operating range of from 1100 nm to 1700 nm.
Two amplifier configurations have been used to amplify wavelength band ranges greater than can be amplified with singular rare earth-doped amplifiers. The first of these is the Raman fiber amplifier which converts laser radiation from a pump laser into another wavelength range stimulated Raman scattering. More specifically, Raman scattering operates on the principle of Stokes light generation, which is downshifted from the optical pump frequency by an energy determined by vibrational oscillation modes in the atomic structure of the fiber. In other words, Raman gain results from the interaction of intense light with optical phonons in the glass, and the Raman effect leads to a transfer of power from one optical beam, or the pump, to another optical beam, or the signal. During a Raman gain, the signal is downshifted in frequency and upshifted in wavelength by an amount determined by the vibrational modes of the glass or the medium.
In operation a pump laser is used to conduct pump radiation through a Raman medium. Signal radation which propagates co-linearly with the pump will be amplified by stimulated Raman scattering, wherby a pump photon is stimulated to emit an optical phonon and also a photon at the same energy and phase as the signal photon. The wavelength range over which amplification occurs is referenced to the wavelength of the optical pump and the bandwidth is determined by the phonon spectra of the Raman medium. A direct consequence of this is that amplification can be realized at any wavelength in an optical fiber by correct choice of the wavelength of the optical pump.
The gain of the Raman amplifier is determined by the Raman gain coefficient, the pump power, the fiber length and the effective area of the optical mode in the fiber. For high gain, a high gain coefficient, a high pump power and a long fiber length along with a small effective area are required. The Raman gain coefficient for silica fibers is shown in FIG. 1. Notably, the gain extends over a large frequency range of up to 40 THz with a broad peak centered at 13.2 THz. This broad behavior is due to the amorphous structure of the silica glass and means that the Raman effect can be used to effect broad band amplification. The Raman gain depends on the composition of the fiber core and can be varied with different dopant types and concentrations within the fiber.
One of the problems generally associated with Raman amplifiers is the requirement of a relatively large pumping power. Raman amplifiers require a significantly higher optical pump power to achieve the same gain associated with Erbium-doped fiber amplifiers. In addition, a significant proportion of the optical pump power can be wasted and unused at the fiber output as a result of the inefficiency of the Raman gain. A significant advantage, however, of Raman amplifiers is the low noise figure associated therewith. More specifically, noise figures close to the quantum limit of 3 dB are possible with Raman amplifiers.
It is known to use Raman fiber amplifiers in conjunction with Erbium-doped fiber amplifiers in transmission systems. However, while the utilization of distributed Raman amplification in conjunction with Erbium-doped fiber amplifiers alleviates the need for high Raman gain, the utility of such configurations are limited to the effective Erbium window, or to other rare earth windows.
A semiconductor optical amplifier is a second kind of amplifier that can also provide gain over the entire operating transmission window of 1100 nm to 1650 nm. For example, semiconductor optical amplifier components based on the material composite of Gax In1-x Asy P1-y can provide gain within the range of 1000 nm to 1650 nm depending on the relative concentration of the constituent elements.
Optical amplification, including amplification affected by a semiconductor optical amplifier, relies on the known physical mechanisms of population inversion and stimulated emission. More specifically, amplification of an optical signal depends on the stimulated transition of an optical medium from an inverted, excited state to a lower, less excited state. Prior to the actual amplification of the optical signal, a population inversion occurs, i.e., more upper excited states exist than lower states. This population inversion is effected by appropriately energizing the system. In semiconductor optical amplifiers, an excited state is a state in which there exists an electron in the conduction band and a concomitant hole in the valence band. A transition from such an excited state, to a lower state in which neither an electron nor a hole exist, results in the creation of a photon or a stimulated emission. The population inversion is depleted every time an optical signal passes through the amplifier and is amplified. The population inversion is then reestablished over some finite period of time. As a result, the gain of the amplifier will be reduced for some given period of time following the passage of any optical signal through the amplifier. This recovery time period is typically denoted as the xe2x80x9cgain-recovery timexe2x80x9d of the amplifier.
In contrast to Erbium-doped amplifiers, or other rare earth-doped amplifiers, semiconductor optical amplifiers are smaller, consume less power and can be formed in an array more easily. Accordingly, semiconductor optical amplifiers are important in applications such as loss compensation for optical switches used in multi-channel optical transmission systems or optical switchboard systems. In contrast to Raman fiber amplifiers, semiconductor optical amplifiers are electrically pumped and as such, provide very efficient gain.
Two major drawbacks are associated with semiconductor optical amplifiers. The first drawback is that the noise figure associated with semiconductor optical amplifiers is significantly high. While all amplifiers degrade the signal-to-noise ratio of the amplified signal because of amplified spontaneous emission that is added to the signal during amplification, the noise figure associated with semiconductor optical amplifiers is extremely problematic. More specifically, the best achievable intrinsic noise figure for semiconductor optical amplifiers is around 4 dB for devices based on multiple quantum well structures, and around 5 dB for devices based on bulk guiding structures. Further, since the optical mock field diameter is very small in semiconductor optical amplifiers with respect to optical fibers, the coupling loss between the two is poor (generally 2-3 dB). As a result, the best achievable noise figures associated with semi-conductor optical amplifiers are typically somewhere between 6 to 8 dB, depending on the device structure and the coupling configuration. A second problem associated with semiconductor optical amplifiers is signal cross-talk resulting from cross-gain modulation. Signal cross-talk arises because the saturation output power of the semiconductor optical amplifier is lower than that of fiber based amplifiers, and because the gain recovery time is on the same time scale as the data repetition rate. Thus, a semiconductor optical amplifier amplifying multiple signals with a combined input power greater than the input saturation power will superimpose cross-talk caused by gain modulation between the relative channels.
The invention relates to an optical signal amplifier that includes a Raman fiber amplifier with a semiconductor optical amplifier. More specifically, the present inventive optical amplifier makes use of the low noise figure typically associated with Raman amplifiers, the significant gain typically associated with semiconductor optical amplifiers and the residual pump power from a Raman amplifier to increase the saturation output power of semiconductor optical amplifier.
In a preferred embodiment, an optical signal amplifier includes a Raman fiber amplifier having an input for receiving an optical signal and a pump radiation, and an output, wherein the pump radiation amplifies the optical signal resulting in an amplified optical signal and a residual pump radiation. The optical signal amplifier also includes a semiconductor optical amplifier having an input coupled to the output of the Raman amplifier for receiving the amplified optical signal and the residual pump radiation, wherein the semiconductor optical amplifier amplifies the input signal resulting in a twice amplified signal, and wherein the residual pump radiation increases a saturation output power of the semiconductor optical amplifier, thereby reducing signal cross-talk in the twice amplified signal.
In addition, two disclosed embodiments of the optical signal amplifier include an optical isolator for preventing a backward propagating amplified spontaneous emission generated within the semiconductor optical amplifier from reaching the Raman fiber amplifier, and a variable optical attenuator for attenuating the residual pump radiation prior to the residual pump radiation reaching the semiconductor optical amplifier.
Other embodiments include an optical communications systems that utilizes the optical signal amplifier, as well as a method for utilization of the composite optical signal amplifier.
One of the advantages of the optical amplifier of the present invention is that it offers the benefit of a relatively large gain in optical signal strength together with a substantially low noise figure and a minimization of signal cross-talk resulting from cross-gain modulation.